14 



GENUS PALLAVICINIA 



The plant has an elongated prostrate thallus with a thick and very 

 conspicuous midrib (Figs. y, A, 8, E). The thallus may reach a length 

 of upwards of 20 cm. with a width of 7 mm. The thallus is usually 

 * forked and is attenuated posteriorly, due to the gradual suppression of 

 the lamina. The midrib itself is practically of equal diameter through- 

 out, and projects strongly on the ventral side. As already stated, the 

 wings of the thallus become gradually narrower toward the base of the 



Pallavicina ra diculosa . 



A. Male plant, x 3. 



B. Cross-section of young conducting tissue, very highly magnified. 



C. Midrib of the thallus, showing the position of the antheridia. 



D. Young ; E, older rhizoids. 



